grafana/k6 · error

format of value must be k=v, received

Error message

format of value must be k=v, received %q

What it means

Returned while parsing K6_BROWSER_SCREENSHOTS_OUTPUT when a comma-separated segment does not contain exactly one '=' (the code splits each segment on '=' and requires exactly 2 parts). It fires for empty segments (trailing comma), segments with no '=', and — importantly — values that themselves contain '=' such as presigned URLs with query strings or base64 padding, because splitting then yields 3+ parts.

Solutions

1. Remove stray segments/commas so every part is exactly key=value
2. Keep '=' out of values: strip the query string from the url= value and pass authentication via header.<name>=<value> entries instead
3. If the presigned URL cannot avoid '=', the parser cannot represent it — request a signature without '=' padding or upload to a URL without query params

Example fix

# before — presigned URL with '=' in the signature
export K6_BROWSER_SCREENSHOTS_OUTPUT='url=https://s3.example.com/b?sig=abcd=='
# after — bare URL, auth moved to a header
export K6_BROWSER_SCREENSHOTS_OUTPUT='url=https://s3.example.com/b,header.X-Auth=abcd'

const v = process.env.K6_BROWSER_SCREENSHOTS_OUTPUT ?? '';
if (v) {
  const bad = v.split(',').filter(p => p.split('=').length !== 2);
  if (bad.length) {
    throw new Error(`segments without exactly one '=': ${bad.join(' | ')} — remove '=' inside values (e.g. presigned query strings)`);
  }
}

When it happens

Trigger: A trailing comma ('url=https://h/,'); a bare token ('basePath'); a URL with a query string ('url=https://h/?sig=abc'); an AWS/GCS presigned URL whose signature contains '=' padding; a doubled equals ('url==https://h/').

Common situations: Pasting a presigned S3/GCS upload URL straight into the env var — its X-Amz-Signature/goog-signature query parameters almost always contain '='. Also stray commas or missing '=' from hand-edited CI variables.

Related errors

• empty header key, received
• format of header must be header.k=v, received
• invalid option
• invalid url
• parsing

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/2fda3f96838f9ca9. Report an issue: GitHub.

Appendix: source

Thrown at internal/js/modules/k6/browser/browser/file_persister.go:52

}

// parsePresignedURLEnvVar will parse a value such as:
// url=https://127.0.0.1/,basePath=/screenshots,header.1=a,header.2=b
// and return them.
//

func parsePresignedURLEnvVar(envVarValue string) (presignedURLConfig, error) {
	ss := strings.Split(envVarValue, ",")

	presignedURL := presignedURLConfig{
		headers: make(map[string]string),
	}
	for _, s := range ss {
		// The key value pair should be of the form key=value, so split
		// on '=' to retrieve the key and value separately.
		kv := strings.Split(s, "=")
		if len(kv) != 2 {
			return presignedURLConfig{}, fmt.Errorf("format of value must be k=v, received %q", s)
		}

		k := kv[0]
		v := kv[1]

		// A key with "header." means that the header name is encoded in the
		// key, separated by a ".". Split the header on "." to retrieve the
		// header name. The header value should be present in v from the previous
		// split.
		var hv, hk string
		if strings.Contains(k, "header.") {
			hv = v

			hh := strings.Split(k, ".")
			if len(hh) != 2 {
				return presignedURLConfig{}, fmt.Errorf("format of header must be header.k=v, received %q", s)
			}

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